Brock Technique of
Strabismus Training
INSTRUCTION MANUAL
KEYSTONE
VIEW COMPANY
www.keystoneview.com
TABLE OF CONTENTS Page
Rationale of Orthoptic Training............................................................... 2
Description of Color depth Strabismus Slides...........................................3
Where to Begin Training; When to Stop Training.....................................6
STAGE I................................................................................................ 6
String Test for Retinal Correspondence.............................6
Common Visual Directions.................................................7
Lid Glow Technique............................................................8
Comparison of Visual Direction in
Normal Correspondence and Panoramic Vision...9
Viewing Distance...............................................................10
Procedures....................................................................... 11
STAGE II................................................................................................ 15
Mirror Testing Technique...................................................17
Procedures........................................................................ 17
Evidence of the Innateness of Stereoscopic Vision.........19
Prism Effect at Given Distances
When Disparity is 30mm....................................... 22
Approach Method with 90mm Separation........................23
Approach Method for Viewing Distances......................... 23
Tables of Angle Subtended for Various Size Images.......24
Performance Tests............................................................ 26
STAGE III................................................................................................. 29
Procedures........................................................................29
STAGE IV................................................................................................. 34
Objectives and Activities................................................... 34
Differential Performance Tests......................................... 36
Procedures........................................................................ 38
Amblyopia................................................................................................43
Procedures.........................................................................43
THE RATIONALE OF ORTHOPTIC TRAINING
1. Normal binocular vision develops in the child according to a definite pattern. (Gesell)
2. The natural (phylogenetic) order of development is from the undifferentiated to the particular, as from an
indeterminate peripheral awareness of "something out there" to precise central fixation and resolution; from
a vague. "Whereness" to discernment of depth differences of less than 1/4 inch at 20 feet (2 seconds of
arc). That is, the order of the growth and refinement of the visual skills proceeds from the peripheral to the
foveal.
3. The surrender of binocular skills proceeds in the reverse order in which they were acquired: first the foveal,
last the peripheral.
4. In STRABISMUS the normal binocular skills have been lost and in many instances even basic binocular
vision (normal correspondence) has been abandoned; reversions, characterized by constant or alternant
or even simultaneous monocular vision, occur.
5. A sound corrective program follows the order of growth of normal binocular vision.
An orthoptic program based on the above classifications will fall into the following categories:
STAGE I. Establishment of normal correspondence.
STAGE II. Restoration of simultaneous perception.
STAGE III. Development of peripheral stereopsis.
STAGE IV. Development of macular stereopsis.
The Color Depth Strabismus Services as conceived by Dr. Brock has been redesigned to eliminate
the necessity of projection of the slides. With the use of a light box, such as the Franzblau viewer (catalog
#2195), the slides are easily presented to the patient and can be manipulated by the doctor or technician.
At a reading distance the patient may work with the slides directly.
The slides themselves are of a size to easily be viewed from 10in. to 20 ft. They are interesting to
youngsters, hold their attention, and arouse a desire to achieve.
Color Depth Strabismus Procedures
Dedicated
To the Memory of
Frederick W. Brock
And to his work in Strabismus Technique
DESCRIPTION OF COLOR-DEPTH STRABISMUS SLIDES
Slides
VD-BC-Consists of opposing arrows set perpendicular to each other with a horizontal scale running between
them. VD slides are routinely used to find the most favorable viewing distance or favorable target separation.
See procedure B Stage II for detailed instruction.
IA- A black rabbit with a carrot (carrot is red, top green). The rabbit is used as a gross central fixation target
with rings IB and IC. The red carrot is seen through the green filter and the green top through the red filter.
This arrangement provides a check on suppression.
IB-Ring, red
Diameter, 6 inch
IC-Ring, green
These rings are especially useful in developing peripheral stereopsis. May be used with following slides
and slide combination: IA, 3A, 6BC, 9BC, and 10BC.
2A-Tree, black. Used with 2BC and 5BC as a central fixation target and plane of reference for stereoscopic
judgments.
2B-Rabbits left one red, right one green. Show details.
2C-Rabbits left one green, right one red. In silhouette.
A highly effective means of eliciting stereopsis in the examination. Used for developing peripheral
stereopsis. Suitable for developing quick and accurate stereo judgment. May be used with 2A, L and ¯
slides.
3A-Red bird and green lamb. Used as a check on suppression with 3BC and 4BC.
3B-Two interlocking rings, upper green, lower red.
3C-Two interlocking rings, upper red, lower green.
Useful in developing both gross and refined stereopsis. Rings may be combined to move in the same or
opposite directions. Used with: 3A, 4A,L.
4A-Elephant and cat. Stereoscopic. Much used with 3BC, 4BC, 6BC,9BC, 10BC, as reference points for
stereo judgment.
4B-Two dogs, upper green, lower red.
4C-Two dogs, upper red, lower green.
When slides are disparated, dogs move in opposite directions. Excellent for developing range and accuracy
of stereopsis. Combine with: 3BC, 4A, 10BC, L.
5B-Green panda, red bear.
5C-Red panda, green bear.
Panda and bear move in opposite directions. Because of subject appeal to small children, especially effective
in eliciting stereopsis in young patients. Useful in developing speed and accuracy of perception. Used with:
2A, 3A, 4A, and L.
6B-Red ring, green space ship.
6C-Green ring, red space ship.
Ships and rings move in opposite directions. Combine central and peripheral training. Used with IBC, 4A,
and 9A.
8A-Black ring. Diameter, 50mm.
Used with: 9BC, 10BC for paramacular fusion and plane of reference in stereo training.
9A-Black ring. Diameter, 66mm.
Used for peripheral fusion and reference plane with: 3BC, 6BC, 9BC, 10BC.
9B-Red duck.
9C-Green duck.
Three-dimensional effect pronounced. Appealing to small children. Much used for eye-hand coordination
training. Used with: IBC, 4A, 8A, 9A.
10B-Red engine.
10C-Green engine.
Boys like to drive an engine. Used with: IBC, 4A, 8A, 9A.
14B-Russian glacier, red.
14C-Russian glacier, green.
The dark, massive bulk and sharp contours of the foreground, silhouetted against the whiteness of the vast
glacial fields covering the side of the towering mountain, together with the effect of great distances, enhanced
by the "railroad track", produces a training situation having many possibilities. Used with: 2A, L, 14BC.
ÿ- Oblique white lines on red background. Used in suppression training in conjunction with almost any slide
combinations desired.
L-Vertical row of black letters. A binocular fixation target. Demands precise bimacular fixation. Afford precise
spatial judgments by orienting the individual letters with different objects in the scene. Used with: IBC, 2BC,
3BC, 4BC, 5BC, 14BC.
R-Red (ruby) plastic sheet. Used as a performance check on stereopsis and malingering.
Note: Scale on the small A slides, while used mostly for reference, can be read directly in Diopters at 16
inches.
Values are to be added or subtracted depending on the setting of the two moveables slides that is. Add
values if slides are on either side of zero. If they are both on the same side of zero the values should be
subtracted.
GENERAL
Where to begin training.
The majority of untrained strabismics can demonstrate peripheral stereopsis. In such cases training should
begin with Stage III.
When there is a severe central suppression or alternation, it may be necessary to start with Stage II.
When normal correspondence is lacking, it will be necessary to begin with Stage I.
When to stop training.
When normal correspondence cannot be established. When a plateau is reached, discontinue
temporarily. When no further progress seems possible, give extended vacation. When patient (and parents)
fail to cooperate, discontinue until such a time as sufficient interest is shown.
STAGE I
String Test for Retinal Correspondence
1. Fit red color filter on right eye, green on the left. Let tester hold one end of a white string (about three
feet long) to the bridge of his/her nose. The examiner grasps the other end of the string tautly and holds a
small light source, a penlight, ophthalmoscope or retinoscope, against the string.
2. Testee is asked to fixate light and is urged to see two strings (physiological diplopia). When necessary,
use sufficient vertical prism to produce diplopia.
3. Testee is to report the color of each string.
If the green string proceeds from the right eye and the red string from the left eye, correspondence is normal.
If the red string proceeds from the right eye and the green string from the left eye, correspondence is not
normal.
STAGE I
Common Visual Directions
Normal retinal correspondence is prerequisite to the correction of strabismus.
When the right and left images can be perceived as occupying the same place, the eyes are said
to have common visual directions (Normal Correspondence). When the right and left images cannot be
superimposed, although perceived simultaneously, normal retinal correspondence is absent.
The AFTER-IMAGE TEST is the most widely used means of determining correspondence. This test,
which is concerned only with correspondence in the central field, is too well known to require description.
The LID GLOW Technique described below is probably the most informative test for retinal correspondence.
The BROCK COLOR-DEPTH SLIDES also contain much material for investigating and training the sensorial
retinal relationship.
LID GLOW TECHNIQUE
1. Let the patient fixate a small black dot placed in the center of a light gray card held by him/her.
2. Flash a small light-retinoscope, ophthalmoscope or pencil flash light-against the closed lid of the deviating
eye. Ask him/her where he/she sees the "glow" in relation to the black dot. It will be found to lie on the axis
of the turned eye.
3. Instruct him/her to turn his/her head as he/she fixates the dot, until he/she sees the glow around the dot.
Success indicates that correspondence is normal; failure indicates non-correspondence.
Interpretation
Under the condition of the test the response become highly significant of the existing cortical organization
of vision.
(a) The glow, experienced through the closed lid, is diffused and consequently is not seen as coming from
any specific direction. Hence the report that it lies to the right or to the left of the dot indicates the mental
direction assigned to the stimuli receives from the turned eye.
(b) If by turning the head the dot can be brought within the glow, it means that "in principle", at least, the
mind retains the kind of relatedness known as fusion or common visual directions.
(c) If on the other hand the glow and the dost seem to evade each other, no matter where he/she turns
his/her head, it can be said that the eyes act as independent visual mechanisms. In other words, the eyes
do not have common visual directions.
Actually, the individual has developecd two separate visual systems integrated in a kind of "binocular" vision,
in that both are used simultaneously and the monocular fields are neatly joined at the midline.
COMPARISION OF VISUAL DIRECTIONS
IN NORMAL CORRESPONDENCE AND PANORAMIC VISION
Fig. I illustrates how the visual direction of an object not foveally fixated is determined.
A is the binocular fixation object. Hence the images of F-1 and F-2 fall on the respective foveas. Since B
is situated to the right of A, B-1 falls on the temporal retina of the left eye and B-2 on the nasal retina of the
right eye. In common experience, when the images of B so fall, B will be perceived in the above relation
to A, that is, to the right of A.
Fig.2. The VD slide is used to demonstrate the principle involved in Procedures B, Stage I. The green arrow
(G) corresponds to the green panda and the red arrow (R) to the red bear. The green filter is before the
right eye.
The VD slide is used because the arrows correspond to the diplopic images in the familiar vertical
prism dissociation test.
Fig 2 shows the retinal position of the image of the green arrow (G) in the non-fixating exotropic eye. Since
the left eye is turned to the left of G the image falls on the temporal retina and in accordance with common
experience (see explanation, Fig.1) will be perceived to the right of R. This is known as "crossed" diplopia.
As will be readily deduced, the opposite situation obtains in esotropia; here the diplopic images are
"uncrossed". The above explanation is valid only when retinal correspondence is normal.
Fig. 1.
Fig. 2.
If normal correspondence is not present (panoramic vision), the arrow will be seen in vertical alignment
or the green arrow somewhat to the left of the red arrow, in exotropia. In esotropia the arrows will be in
vertical alignment or the green arrow responses are referred to in the literature as "paradoxical" diplopia,
i.e. contradictory to those received when correspondence is normal. The analysis is presented in Stage I,
Procedures A, Step 3 and the second paragraph on the following page (procedure B).
Applying the above explanation to the panda and the bear (5B), it is obvious that in left exotropia
the panda will be seen as to the left of the bear regardless of the actual positions of the two animals,
because in common experience anything seen via the left eye must of necessity be in the left field.
Needless to say, the same condition obtains in right exotropia. In esotropia the panda will be seen
to the right of the bear because the field of the left eye lies to the right of that of the right eye.
VIEWING DISTANCE
The viewing distance is usually near the crossing point of the eyes.
To attain binocularity, compensating lenses or prisms are sometimes indicated in early stages of
training: plus and/or base out prism in esotropia, minus and/or base in prism in exotropia.
Table below will show crossing point when the angle of squint is known.
10 13
20 6.5
25 5
30 4
35 3.5
40 3
45 2.5
STAGE I
Procedure A: Testing and Training
Purpose: To elicit the LUSTER phenomenon.
Note: Color fusion indicates that normal correspondence is not entirely lost.
Instrumentation: Franzblau Projection Box Viewer.
Slides: None
Angle in Crossing point
Degrees: in inches:
Viewing distance: VD slide will indicate.
Color Filters: Green over dominant eye.
In right esotropia the patient should be instructed to look to the right side of the field, in left esotropia,
to the left side.
In exotropias, the above instructions are reversed.
Let trainee cover and uncover, with his/her hand, the eye wearing red filter, and report whether he/she
sees any changes in the green color. If the report is negative, let him/her cover and uncover the dominant
eye. Covering and uncovering one eye enables the patient, who does not have normal correspondence,
to tolerate binocularity for short intervals.
If no fusion of colors has taken place, try:
a. Changing the viewing distance.
b. Rapidly turning viewers light off and on.
Interpretation of unsuccessful response:
1. If the color seen by the non-dominant eye is not perceived when both eyes are uncovered, suppression
is probably total.
2. Alternating strabismus is indicated when the colors are seen in rapid sequence. True alternation should
not be confused with retinal rivalry. In alternating strabismus, the shifts from one color to the other are abrupt
and usually follow each other in fairly quick succession. In retinal rivalry, the changes are gradual-one color
receding slowly as the other advances.
3. Normal correspondence is lacking when the field is half red and half green. The "half red and half green"
phenomenon indicates that the right and left visual fields are perceived independently, but simultaneously,
i.e. vision has become panoramic. When this condition exists, correction of the strabismus by orthoptics
will prove exceedingly difficult.
Begin with Procedure B and Lid Glow Technique.
Procedure B
When vision has become panoramic, binocularity may sometimes be aroused by reversing the fields
and then showing the subject that his/her visual directions do not correspond with reality.
In panoramic vision, the direction in which each eye sees is independently related to the body position.
If, for example, the left eye is exotropic, whatever is perceived via that eye is ipso facto assumed to be
located to the left of the midline or to the left of that which is seen via the right eye. Consequently, if a situation
is arranged where an object seen by the left eye is placed to the right of that which is seen by the right eye,
the subject will still perceives the object belonging to the left eye as being to the left.
Now if he/she places his/her left index finger on the object seen by the left eye and the right index
finger on the object seen by the right eye, he/she discover that he/she is locating the "left" object to the right
and the "right" object to the left-from the fact that his/her hands have crossed. Since this is contrary to the
visually perceived directions of the respective objects, a condition has been created which calls for reorientation.
Testing
1. Place green filter over right eye, with slide 5B on viewer (panda will be to the left). Ask "To which side of
the bear do you see the panda, right or left?" pointing, the fingers will locate the objects in direction opposite
to where they are perceived visually. He/she discovers the contradiction when he/she finds that the hands
crossed in the process. He/she comes to the astounding conclusion that one of the senses has deceived
him/her.
2. Remove color filters and let him/her see for him/herself that his/her eyes have played him/her false.
Unbelievable as it is, he/she must accept the incontrovertible fact. To him/her that is an intolerable situation,
which must be corrected. The first, and most important, step toward restoring normal correspondence has
been taken. Success will depend on the determination and persistence of the trainee and the skill of the
technician.
The following individuals slides and combination may be utilized as illustrated in the above procedure, to
teach the necessary intersensory coordination: 2B or 2C, 4B or 4C, 5B or 5C (the following combination
should be disparated to various separations, crossed or uncrossed as the situation demands) 10B-9C, 10C-
9B.
Procedure C
1. Illuminate Viewer. Place patient as far from screen as possible but not more than 20ft. Fit color filters.
2. Find the amount of prism required to produce vertical diplopia. Let patient hold prism or prisms.
If normal correspondence is absent, the images will usually be very nearly in vertical alignment, regardless
of the angle of squint.
3. Introduce lateral prism (15 diopters or more), in addition to the vertical prism.
If normal correspondence is absent, no amount of lateral prism will cause the images to shift.
4. Let patient walk slowly toward the screen (vertical prism in place) and attempt to bring the images into
superimposition.
If monocularity perception is dominant, the images will shift laterally because of the lack of common
visual directions. It should be remembered that the aversion to fusion might not extend to the peripheral
areas. Let patient come within 3ft. of the screen (see Table No.2 for angle subtended at different
distances).
If even partial superimposition cannot be achieved at close range, the indication is that normal
correspondence has been completely abandoned, that vision is panoramic.
Try Procedure D.
Procedure D
Purpose: To attain superimposition of figure, seen by one eye, upon ground, seen by theother eye.
Illuminate Viewer. Translucent and place ¯ slide (used as "ground").
Slides: Place ¯ (used as "ground") on viewer with any one of the slides listed below (used as "figures"): IC,
6C, 9C, and 10C. (14C is more difficult than the others listed and should be used last).
Viewing Distance: Place the patient close enough so that both lines of gaze fall well within the lighted area.
Color Filters: Red on non-dominant eye.
Instruct patient to attempt to see the figure surrounded by oblique lines.
If he cannot achieve, no matter how close he/she comes to the screen, raise filters and let him/her
see that lines surround the figure.
Then let him/her place a pointer on the figure and with the other hand trace the lines all around and
across the finger.
As he/she traces, lower filters and urge him/her to continue to see the lines around and over the
figure.
Kinesthetic reinforcement is a powerful corrective.
In some instance rapidly turning on and off the viewer light is effective.
The establishment of common visual directions or correspondence requires patience and persistence.
Do not continue, however, to the point where patient feels frustrated or has decided that he/she simply
cannot do it.
Make the exercise a game.
Change targets (C slides) frequently to maintain interest.
If achievement is slow, make sure that he/she can do the Lid Glow and the Mirror exerices (see Mirror
Technique Stage II). What these can be managed readily, Procedure D should not prove very difficult.
IMPORTANT. Sometimes during every training session of STAGE I try for peripheral stereopsis (STAGE
III). Patient may pleasantly surprise you by suddenly "remembering" his/her innate stereoscopic ability.
What that day comes, you should both celebrate.
STAGE II
Purpose: To establish simultaneous perception (eliminate suppression) when normal retinal correspondence
is present.
Normal binocular vision may be present in the peripheral areas when suppression exists centrally.
Aversion to fusion is peculiarly a macular phenomenon, which seldom extends to the whole field. This is
shown by the fact that the strabismic will often demonstrate peripheral stereopsis when he/she cannot
simultaneously perceive centrally located dissimilar images such as the bird and the cage. It is important
to find areas where stereopsis is still present, because the most effective method of eliminating suppression
is to extend such fusion areas.
As soon as peripheral stereopsis can be evoked, the exercise under STAGE III should be employed
to the fullest extent. At that point, the exercises under STAGE II become supplementary. They should
be used in conjunction with stereoscopic training whenever they promote simultaneous awareness in areas
where stereopsis has not yet been elicited.
Nearly all strabismics with normal correspondence will show peripheral, and often even central,
stereopsis under appropriate testing conditions, prior to any training whatever. Therefore, STAGE II may
in many cases be bypassed.
Mirror Testing Technique
The purpose is to bring about superimposition of right and left images of familiar but dissimilar objects.
The "magic" of seeing one thing with one eye and something else with the other eye and being able
to move them around at will, is the kind of game that appeals to the imagination of a small child.
Make the test fun.
1) Place the child near a window, at right angle to it, so that the turned eye is on the side of the window.
Let him/her hold a small mirror before deviating eye and against the side of his/her nose. Instruct him/her
to angle it so that he/she sees the window reflected in it.
2) Let him/her hold a small light (seen only by the dominant eye) in his/her other hand and as he/she looks
at the window reflected in the mirror, tell him/her to put the light in the "window". To do this he/she will need
to turn the mirror so that it will be at right angles to the axis of the deviating eye.
If he/she has difficulty in seeing the window and the light at the same time, let him/her hold the mirror in front
of the dominant eye. Now if he/she keeps the light moving, say in small circles, he/she should be able to
see the window and the light at the same time and finally superimposed.
Repeat the above procedure with various objects in the room, advancing from large and undifferentiated
objects to smaller objects having clearly defined but dissimilar contours until he/she can superimpose
"anything".
Ingenuity is required in carrying out this kind of training. Let the child suggest things he/she would like to
put on top of each other. If the game were played outdoors, perhaps he/she would like to put an "automobile"
on top of a house. Or catch his/her dog in the mirror and have him/her "fly" over the trees. This can be even
be fun for the parent whose privilege it will be to assist with the training at home, once the ability to perform
at all has been established.
STAGE II
Procedure A
Use Procedure D, STAGE I. This procedure, which was used to establish normal correspondence, is also
equally well suited to the radically different, but simpler, problem of establishing simultaneous perception.
Procedure B
Color filters: Green filter over dominant eye, before slides are inserted.
Slides: Slide 4A with 3A or 3C, 4C, 6C, 9C, 10C, and slide ¯ are used in this procedure.
Finding the viewing distance:
Insert slide VD. Ascertain that both arrows are seen. Ask the patient to approach the screen (or NV) until
the arrows are in approximate alignment.
In esotropia when the viewing distance is very short, accommodation may be stimulated sufficiently
to reflexly increase the angle of squint. In that case, plus may be added or combined with base out
prism, as a temporary measure.
In exotropia, where the "crossing point" is behind the patient, it may be necessary to use sufficient
minus to stimulate convergence reflexly and/or base in prism.
Place Slides 4A, 4B, ÿ on viewer and ask: "What animals do you see?" "Do you see any lines?" "Where?"
If red-dog or lines are not seen, use pointer or flashing. Let patient trace lines.
Do not continue too long with any combination of slides. Variety adds interest.
Also ask: Which is closer to you, the elephant or the cat?"...the elephant or the upper dog?"
At every opportunity explore the possibility of eliciting peripheral stereopsis.
Remember, if correspondence is normal and perception simultaneous (not alternating), stereopsis can
nearly always be evoked immediately.
Procedure C
Color filters: Red filter on dominant eye, before slide is exposed.
Slides: IA
Viewing distance: Crossing point
i. If he/she cannot see carrot, let him/her use pointer.
ii. When carrot is seen, alter viewing distance, if necessary, to bring carrot top over the carrot.
When the angle of squint is not constant, some fluctuations are to be expected.
iii. Add slide IB. Patient should see carrot and the ring. When successful,
iv. Add IC and disparate slides slightly to bring the ring out in front.
Ask: "Are the rabbit and the ring the same distance from you?"
To stimulate the stereo reflex, move ring in and out with quick, short lateral movements of the slides.
The building of strong peripheral stereopsis is the first order of business in the correction of strabismus,
because of this powerful binocular attraction of corresponding peripheral areas. Furthermore, angular extent
of Panum's areas in the peripheral allows for much more retinal slip than is permissible at the fovea, without
producing diplopia. Consequently, fusion is usually obtained much more easily peripherally than centrally.
EXPERIMENTAL EVIDENCE OF THE INNATENESS OF STEREOSCOPIC VISION
Slides: 1A, 1B, 1C.
1) Observer wears red color filter over right eye, green over the left eye. He/she remains at a constant
viewing distance and maintain fixation of the rabbit.
2) a. When the Red ring is disparated to the right of the green ring (crossed disparity) the perceived ring
approaches the observer and becomes progressively smaller.
b.When the red ring is disparated to the left (uncrossed disparity), the perceived ring recedes and
becomes progressively larger.
These size changes contradict common experience but are characteristically accepted without question.
Considerations
The size of the projected rings did not change. The observer remained at the same distance from
the rings and maintained fixation on the rabbit. Therefore, no changes in the size of the retinal images or
the angle of convergence or accommodation occurred.
In common experience, these components undergo greater or lesser changes, when the distance
of the object of regard is altered. Since here they remained constant, they could furnish only contradictory
evidence of changes in localization. Nevertheless, the non-stereoscopic clues are rejected and the stereoscopic
clues accepted, without question.
Two questions present themselves:
Why does the phenomenal ring undergo any change in size, when the retinal image has not changed?
Why are the sizes changes contrary to common experience?
The suggested answers point to the innate character of stereoscopic perception.
In common experience we do not consciously interpret retinal image size changes with any degree
of accuracy. Nevertheless, when an object is brought closer we, at least, never experience a reduction in
perceived size, nor when it is moved farther away, does it seem to get larger. Again, why does everyone
who has normal binocular vision accept automatically the contradictory size changes that accompany the
three-dimensional ring movements?
Suggested Explanation
The sensation of having the images remain the same size while they move across the retinas as
though the object had moved closer or farther away creates a dilemma. The monocular evidence of image
size indicates that the ring has not moved; the binocular evidence of disparity answers that the ring has
moved.
This cannot be. The organism can take no effective action until these contradictions are reconciled.
The solution? The monocular evidence is brought into agreement with the binocular, assuming that the ring
has shrunk or expanded. This decision is un-deliberated and instantaneous. There is no mental debate
concerning how an object may be made to shrink or expand, simply by moving it closer or farther away.
The evidence for the primacy of stereoscopic binocular vision could hardly be stronger.
Application
Why all this attention to the innate character of n ormal binocular vision? Because of the bearing it
has on the possibility of restoring binocular vision by orthoptic means.
If normal binocular vision is innate, the problem becomes essentially one of reintegration. The rationale
of the orthoptic program will be to evoke and integrate the visual skills that constitute normal human vision.
Purpose: To develop stereoscopic perception in the peripheral field.
Rationale: The extrinsic muscles are fully developed at birth and extreme convergence may be maintained
for several seconds at six months. But the foveas are structurally undeveloped at birth; at six months the
visual acuity is 20/200 or less and does not reach 20/20 for nearly five years. Since foveal fixation must
wait for foveal acuity, it follows that binocular vision develops peripherally several years before precise bifoveal
fixation is possible. This may explain why so many young strabismics will demonstrate peripheral stereopsis
prior to any training, although they are not capable of putting the bird in the cage.
It seems remarkable that strabismic training has not generally followed the developmental order but has
insisted upon forcing the establishment of central bifixation before awakening the more primitive binocular
functions, because:
1) Peripheral stereopsis can be induced with comparatively little difficulty in practically all strabismics
who do not have panoramic vision, i.e. those who do not see the screen half red and half green.
2) The establishment of peripheral fusion furnished support for precise coordination in the central
areas. Burian's experiments showed that even the vertical displacement of the monocular peripheral
stimulus was sufficient to break central fusion. How potent must peripheral fusion be in inducing and
maintaining central fusion?
3) The early stages of training should be concerned with functions that do not require precise
coordination. Fusion is usually obtained with less difficulty in the periphery than at the fovea because
the sensitivity to diplopia varies inversely as the size of Panum's area.
BASIC PROCEDURE
Disparate, peripheral targets are combined with central stationary targets that do not require precise bifixation.
The viewing distance used depends to some extent on the angle deviation. As a rule, the viewing distance
should not exceed the distance at which the lines of gaze cross (crossing point), i.e. the greater the deviation,
the smaller the image.
PRISM EFFECT AT GIVEN DISTANCES WHEN DISPARITY IS 30MM.
I. Equivalent prism effect when given amounts of plus is added binocularly (1.00D is assumed to be equal
to 4 prism diopters, on the basis of an average AC/A ratio of 1:4).
II. Total prism effect.
I II III
Dist. Prism Plus. Prism Total Prism
40"------3 1.00------4 7
32"------4 1.25------5 9
26"------5 1.50------6 11
20"------6 2.00------8 14
16"------8 2.50------10 18
13"------9 3.00------12 21
10"------12 4.00------16 28
8"--------15 5.00------20 35
6"--------20 6.00------24 44
When image is closer than plane of regard, the effect is base-out. When image is beyond plaen of regard,
the effect is base-in.
If more BI stimulation is desired than is available by disparating the slides, add minus. Calculate effect of
minus as above.
If patient's AC/A ratio is known, it should be used in calculating prism effect induced by lens.
APPROACH METHOD
Preset targets IBC with 90mm separation
Instruct trainee to (1) black away until he/she can fuse targets, (2) walk forward until he/she experience
diplopia, (3) back away to point of recovery, (4) repeat 2 and 3. Every effort should be made to maintain
fusion and to recover it. Do not continue to point of visible fatigue or lowered performance.
Viewing Prism
Distance Diopters
20'--------------------------------------- 1.5
16'--------------------------------------- 1.8
14'5"------------------------------------- 2.3
12'---------------------------------------- 2.5
10'---------------------------------------- 3
8'------------------------------------------ 3.5
7'------------------------------------------ 4
6'------------------------------------------ 5
5'------------------------------------------ 6
4'------------------------------------------ 7
3'5"--------------------------------------- 8
36"---------------------------------------- 10
30"---------------------------------------- 12
24"---------------------------------------- 15
20"---------------------------------------- 18
16"---------------------------------------- 22
The following tables show which areas of the retinas are being stimulated, given the diameter of the target
and the viewing distance (VD). The approximate horizontal dimensions of Panum's area are appended to
Table No.1.
Table No.1
Ratio diameter to VD Arc degrees subtended Panum's arc minutes subtended
1:1 54 48
1:2 27 24
1:3 18 16
1:4 14 12
1:5 11 10
1:6 9 8
1:7 8 7
1:8 7 7
1:9 6 7
1:10 5 6
1:14 4 6
1:18 3 6
1:27 2 6
1:54 1 6
Table No.2
Angles subtended by 10" wide light box.
Distance. Inches Arc degree
12 45
16 35
18 31
24 24
30 19
36 16
48 12
60 10
72 8
84 7
96 6
108 5.3
Peripheral Stage III
Macular Stage IV
Peripheral
Table No.3
Angle Subtended by
IBC and 9A
Dist. Inches Arc Degrees
6 54
8 36
12 27
16 18
18 16
24 12
32 9
36 8
40 7
48 6
60 5
72 4
96 3
144 2
Table No.4
Angle Subtended by 120mm target
Dist. Inches Arc Degrees
2 100
3 76
4 61
5 50
6 58
7 37
8 32
9 29
10 27
12 22
13 55
15 18
18 15
24 11
36 7
48 6
60 5
72 4
96 3
144 2
Peripheral Stage III
Macular Stage IV
Peripheral Stage III
Macular Stage IV
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